We propose to utilize the unique genetic system found in the fungus gnat, Sciara coprophila, using methods from cytology, genetics, and molecular biology to study polytene chromosome architecture. Specifically, we will prepare DNA clones from the Sciara genome and select clones adjacent to the ribosomal DNA in order to determine the number of gaps between the tandem ribosomal DNA repeat units. By use of in situ hybridization to translocation polytene chromosomes this result will be utilized to elucidate if the interbands between the X-heterochromomeres contain non-ribosomal DNA sequences. Hence we will begin to align a DNA sequence map with the cytogenic map. In the future we will continue this study to analyze the DNA comprising the X-chromosome "controlling element" which is located in between ribosomal DNA units at heterochromomere H2. In a second direction we will use purified DNA from a DNA puff obtained by BudR labelling and CsCl centrifugation to select for DNA clones containing these sequences in our gene library.